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The summer of 2025 in Europe highlighted the growing impact of extreme heat. According to the Copernicus Climate Change Service (C3S) Climate Bulletins, successive heatwaves in June, July, and August pushed “feels-like” temperatures to dangerous levels in parts of the continent. These recurring events highlight a clear, data-backed trend: a rising number of heat stress days, which affect not only comfort but also health, productivity, and wellbeing. To help individuals, organisations, and decision-makers assess these impacts, C3S has developed Thermal Trace – a new app which provides near real-time insights into heat and cold stress. In this Observer, we take a close look at the Thermal Trace app.
The pattern of growing health impacts from extreme heat is echoed globally. New findings published by The Lancet Countdown reveal that rising global heat is now killing one person every minute around the world. The health impacts from climate change are increasingly evident. As temperatures climb, exposure and vulnerability are converging in ways which cost lives every day.
In this context, Thermal Trace provides a valuable tool. This intuitive application turns more than eighty years of Universal Thermal Climate Index (UTCI) data into clear, actionable insights on heat and cold stress, presented through maps, charts and downloadable tables which are ready to use in briefings, dashboards and media stories. In doing so, it helps citizens, journalists, planners and public-health professionals make sense of evolving risks and prepare more effectively for the impacts of extreme temperatures.
“Thermal Trace was created to provide crucial insights into heat and cold stress, which are key factors for human health as heatwaves become more frequent and severe. The app provides quick, free and visually engaging access to global maps and charts on thermal stress,” said Rebecca Emerton, Scientist at the European Centre for Medium-Range Weather Forecasts (ECMWF) and C3S coordinator for the app’s development.
What Thermal Trace does
Thermal Trace is a web application developed by C3S at ECMWF which allows users to explore, visualise, share and extract more than 80 years of global heat and cold stress data in just a few seconds. It draws primarily on ERA5-HEAT, a UTCI dataset derived from ERA5 reanalysis, updated in near real time, with a typical latency of about five days. This near-real-time coverage means the app is ideal for unpacking recent events, benchmarking them against the 1991–2020 reference period, and tracking changes over time. The app is a powerful tool for past event analysis and visualisation, but it does not provide live warnings or forecasts. For real-time safety advice, users should always consult national meteorological and public-health services.
At first launch, Thermal Trace allows to explore either cold or heat stress data; it then opens on a global map showing the peak heat (or cold) stress on the latest available day. From there, you can choose daily, monthly, seasonal or annual periods; pick locations by clicking on the map, typing a place name, using your device’s geolocation, or even drawing a custom region; and switch effortlessly between a set of clear visuals:
- Maps of the chosen metric (for example, peak heat stress in June 2025).
- Time series, which reveal how a location’s stress metrics evolve through time.
- Heatmaps which provide a compact “at-a-glance” view of annual or seasonal patterns reminiscent of climate stripes.
The app includes post-processed thermal comfort data which are directly relevant for communicating on heat-health episodes, such as the number of heat stress days, ‘tropical nights’ and cold stress days, alongside daily through to annual maxima and minima. For quick analysis, the data behind every chart can be downloaded for use, while the full dataset is accessible via the Climate Data Store for more technical workflows. An interactive tutorial in the app helps first-time users find their way around the app’s functions.
The UTCI: a health-relevant ‘feels-like’ measure
The UTCI expresses a feels-like temperature in °C and categorises heat stress into moderate (26-32°C), strong (32-38°C), very strong (38-46°C) and extreme (>46°C). For cold stress, UTCI defines categories from slight (9 to 0°C) down to extreme (below -40°C). By focusing on the human body’s response to weather, UTCI is more representative of potential health outcomes than air temperature alone, especially during humid heat or cold, windy conditions. (Read this article to learn more about heat stress and how it is measured.)
For daily selections, Thermal Trace shows hourly variations in both air temperature and feels-like temperature, including the two days before and after the selected date. This is particularly helpful for reconstructing on-the-ground exposure during specific events (for instance, a heatwave day in Córdoba where air temperature peaked around 40.2°C while the feels-like reached ~43.3°C). For monthly, seasonal or annual views, additional charts are available showing the number of heat stress days, tropical nights and cold stress days. These help users quantify the duration and intensity of events, which are two key drivers of health risk often overlooked in simple maximum-temperature summaries.
A “time machine” for case studies
Thermal Trace also functions as a time machine for climate communication and research. For example, you can instantly pull up the 1962/63 “Big Freeze” in Europe to visualise how seasonal minimum feels-like temperatures deviated from average across Europe and highlight the years which stand out most in a given location. Equally, you can map the number of days with at least strong heat stress or cold stress in a given year, for example, the record-warm 2024, and pair that with a long-term time series for a city like Athens, tracking the annual counts of very strong and extreme heat-stress days since 1940.
“Estimates show that there were almost 63,000 heat-related deaths in Europe in 2024,” said Emerton. “The data behind Thermal Trace show a clear signal: heat stress is increasing in Europe, which will have impacts on public health.”
Designed for reliability and speed
Behind the scenes, Thermal Trace employs Zarr, an open-source format which stores large multidimensional datasets in directly accessible pieces, as part of the ECMWF Data Stores strategy to deploy a cloud-ready ARCO Data Lake. Combined with the upgraded Climate Data Store infrastructure, this architecture allows the app to render maps and charts quickly by loading only the data required for the user’s current view, rather than moving entire files across the network. The result is an experience which supports rapid exploration at global scale without sacrificing reproducibility or traceability, similar to the ERA Explorer app.
As previously mentioned, Thermal Trace is not a real-time alerting tool. Its strength lies in helping users understand and analyse thermal stress, including how factors such as humidity, wind, and solar radiation can make the feels-like temperature differ greatly from the actual temperature. It is designed for event reconstruction, comparison and trend analysis rather than for issuing immediate warnings. ECMWF provides UTCI forecasts separately, while real-time safety advice remains the remit of national weather forecasting agencies.
How to use the app
- Localised heat briefings: Draw a small region covering the area you are interested in, choose a daily, monthly, seasonal or yearly timeframe, and export the count of strong/very strong heat-stress days plus the tropical nights metric for the period in question.
- Event reconstructions: Switch to the daily map view, and for a heatwave day, for example, use the charts to view hourly feels-like vs air temperature. This helps identify periods of peak physiological strain.
- Long-term planning: Use annual heatmaps to brief decision-makers on trajectories since 1940. The visual similarity to climate stripes makes the message intuitive for non-specialists.
- Comparative risk: Compare your local 2024–2025 data, for example, with neighbouring regions to understand relative exposure.
Related tools and where to find the data
Thermal Trace is the result of months of design and development across departments at C3S and the wider ECWMF, with the support of the ASPECT Horizon Europe project.
Thermal Trace joins other C3S applications, such as Climate Pulse, the interactive Climate Atlas, ERA Explorer and the Global Temperature Trend Monitor. All of these are built to increase the accessibility of C3S datasets. Together, these tools provide a coherent suite for journalists, educators, policy teams and anyone wanting to connect global climate signals to local experience.
The statistics highlight the scale of exposure and associated health risks. Thermal Trace equips its users with the clearest possible view of the extreme conditions which threaten our health.
